Megger MFT1845 Application Note

APPLICATION NOTE

RCD impedance uplift elimination.

Megger Ltd, Dover, United Kingdom www.megger.com

Sales office: 01304 502 101, [e-mail] RCD impedance uplift elimination

Technical support: 01304 502 120, [e-mail] P1

The MFT1845 utilises a brand new loop impedance test technique which provides immunity to the
increased impedance effects that RCDs and RCBOs can have on the measurement.

Applications:

The new MFT 3-wire non-trip loop impedance technique is suitable for use on all low voltage single phase
circuits, where the earth loop impedance of the circuit is required to be measured through RCD or RCBO
devices.

The problem:

There is a requirement in electrical installations which stipulates that electricians must verify the loop
impedance of all electrical circuits. The loop impedance must be low enough to ensure every circuit
breaker protecting a circuit from over-current, both line to neutral and line to protective earth, will
disconnect in the required time. These requirements are drawn directly from the IEC 60364 regulations
and from domestic implementation of these regulations. On a typical 32 A type B circuit breaker this value
should be less than 1.37 Ω. This is then multiplied by 0.8 to provide a safe margin for error, giving a loop
impedance of 1.1 Ω. We will come back to this later.

Many of these circuits are also protected by an RCD or RCBO downstream of the circuit breaker. The
most common of these, the 30 mA RCD or RCBO can influence the loop impedance measured by the
MFT by increasing the measured value by more than 0.5 Ω, the value returned is significantly higher than
the expected or calculated value. This can easily push the measured value over the limit, as below:

Basic circuit resistance (continuity) = 0.7 Ω
Measured loop impedance (L-PE) = 1.2 Ω (remember the limit is 1.1 Ω)
Uplift created by RCD or RCBO = 1.20 – 0.7 = 0.5 Ω

This uplift of the circuit impedance pushes the measured level over the acceptable limit and now presents
a significant problem for electricians.

The uplift is not present on all RCDs or RCBOs and can vary greatly on those devices where it does exist.

APPLICATION NOTE

RCD impedance uplift elimination.

Megger Ltd, Dover, United Kingdom www.megger.com

Sales office: 01304 502 101, [e-mail] RCD impedance uplift elimination

Technical support: 01304 502 120, [e-mail] P2

The cause:

A typical circuit for an RCD is shown in Figure 1:

Figure 1. Typical RCD circuit.

Coils (A) and (B) are responsible for creating a current in the ferrite core of the RCD. When the current
in (A) and (B) differ, the sense coil (C) detects this current, if the difference is large enough the relay
disconnects the supply. It does not matter how much current is flowing in the L-N circuit, just the difference
is significant. Too much difference between (A) and (B) will trip the RCD.

The simplified circuit in Figure 2 shows just the 15 mA current induced by the MFT in the L-PE during a
non-trip test.

Figure 2. Simplified circuit

This 15 mA current is not enough to trip the RCD. This is a tiny test current, which is why the non-trip test
is so difficult for instrument manufacturers to produce successfully.